Comets formed in the outer reaches of our solar system early in its development. They are made of ice and dust, materials from the original nebula. Comets periodically pass close enough to the Sun to heat up and release a long tail of dust and gas. Nucleus of Comet Halley, courtesy of the European Space Agency and the Max-Planck Institute for Aeronomy.

Beyond the Gas Giants

Planetesimals that have not had enough time to accrete into planets populate the Kuiper belt, which extends beyond Neptune. Some scientists consider Pluto to be a large member of the Kuiper belt, rather than a planet. The Oort cloud, which envelops our solar system and may extend 30 trillion kilometers away from our Sun, contains icy planetesimals. Comets come from the Oort cloud and the Kuiper belt.